In general terms, the present invention is related to an electrical power transmission system using superconductors which is compatible with conventional transmission systems.
As is known, superconductors are metals, alloys, oxides, and, in general, compounds which, below a temperature normally referred to as the critical temperature, show a fall in resistivity to practically zero values.
In particular, a superconductor will remain superconducting only below its critical temperature, below a critical magnetic field, and below a critical current density.
Superconducting materials may be of the low-temperature type, which are generally metals such as alloys of niobium and titanium, or of the high-temperature type, which are generally ceramics such as those based on bismuth, strontium, calcium and copper oxides (BSCCO) or yttrium, barium and copper oxides (YBCO).
Reference may be made, by way of example, for one of these materials and for its preparation, to the description in European Patent EP 646 974 held by the present Applicant.
In the field of superconductors and for the purposes of the present description, the term xe2x80x9clow-temperature superconducting materialsxe2x80x9d denotes materials having an operating temperature of the order of 4xc2x0K (approximately xe2x88x92269xc2x0 C.), and xe2x80x9chigh-temperature superconducting materialsxe2x80x9d denote materials having an operating temperature of the order of 70-77xc2x0K (approximately xe2x88x92203/xe2x88x92196xc2x0 C.).
In order to operate at these temperatures, these superconductors are cooled with suitable coolant fluids, such as liquid helium for the low temperatures and liquid nitrogen for the high temperatures.
For the purposes of the present description, xe2x80x9cconventional cablexe2x80x9d denotes a non-superconducting cable using electrical conductors with non-zero resistance, in particular a cable which has at least a significant portion with characteristics of non-zero electrical resistance. An electrical power transmission or distribution network generally comprises a set of connecting lines consisting of cables or overhead lines, connected in different ways (in terminal load, loop, or mesh configuration) and capable of carrying energy between units connected to interconnection nodes (of the connecting lines of the network) or to terminal nodes of the network, such as sub-stations supplied by electrical power plants, transformer stations and user loads.
Transmission networks may occasionally be subjected to overcurrents, in other words currents having a value higher than the operating value, which occur in the presence of faults and particularly in the presence of short circuits of the equipment and particularly of the lines. In the cables, these overcurrents may cause not only electrodynamic forces capable of damaging parts not securely fixed to the structures, but also an excessive temperature rise which, if persistent, may result in the burning of insulators and fires in combustible materials close to the insulators (transformer oil, for example).
In installations with conventional networks, overcurrent protection is provided by the use of automatic circuit breakers which, by means of an automatic cut-out and reconnection device, open the circuit at a current value equal to a set value and reclose the circuit when the overcurrent ceases.
For the protection of these circuit breakers or other equipment present in an installation, such as transformers, it is possible to use, among other systems, current limiting devices which may be of the induction or resistance type.
The current limiter, installed in series with the equipment to be protected, has a low impedance during normal operation, but when an overcurrent occurs in the network it increases its impedance in such a way as to limit the current to below a threshold value so as not to damage the circuit breaker or transformer. There are known overcurrent limiters, comprising inductances, which make use of the superconductivity characteristics of the materials. Under normal conditions, these limiters, or parts of them, are in a superconducting state and are designed in such a way that they have a low impedance. In the presence of overcurrents, they leave the superconducting state and behave in such a way as to have a high impedance.
Limiters of this type are described, for example, in the patents U.S. Pat. No. 5 140 290, U.S. Pat. No. 5 546 261 and EP 336 337.
The book xe2x80x9cImpianti elettricixe2x80x9d, by Filippo Tiberio, Published by Vanini, Brescia, 1953, describes, for conventional non-superconducting networks, the use of reactance coils which are connected either to the busbars (in series between two sections of bar) or to the lines (in other words between the bars and the lines departing from the power plant).
The Applicant has observed that superconducting cable installations are typically intended to be provided within a conventional network, for example by replacing a conventional cable with a superconducting cable between two nodes of the network, or by inserting a new section.
The Applicant has observed that the problem of the compatibility between transmission systems using superconductors and transmission systems using conventional conductors has not been tackled in the prior art.
In particular, the Applicant has tackled the problem of the behavior of transmission systems using coaxial superconducting cables inside a conventional network in case of a short circuit.
The Applicant has noted that the introduction of a coaxial superconducting cable into a network might lead to an increase in the value of the short-circuit current in the branch in question as a result of the lower value of the characteristic impedance of the coaxial superconducting cable by comparison with that of a conventional cable.
It has also noted that the line comprising the coaxial superconducting cable, having a lower characteristic impedance than that of conventional lines, forms a preferential path for the short-circuit currents, involving the lines close to it which might have to withstand a higher current than a conventional line.
The low characteristic impedance of coaxial superconducting cables is due to their low resistance and also to their low reactance. The latter value is the one which has most effect on the absolute value of the impedance. The reactance of a coaxial superconducting cable is low owing to its coaxial structure, which comprises a phase superconductor and a return superconductor which carries in the opposite direction to the phase conductor a quantity of current equivalent to that carried by the latter. In conventional non-coaxial cables, however, the reactance is a function of the geometrical characteristics of the cable and also of the relative positioning of one cable with respect to the others.
At this point, the Applicant tackled the problem of ensuring the compatibility of a coaxial superconducting cable, in the presence of overcurrents, with the whole network.
The Applicant has also observed that short circuits create problems for superconducting cables. In particular, it has observed that in the presence of a short circuit the superconductor passes from the superconducting state to the state of normal conduction, in other words the resistive state; in this state, the emission of heat by the Joule effect increases considerably, and consequently there is an increase in the temperature of the cable with potential evaporation of the coolant liquid. When normal operating conditions are restored, in other words at the end of the short circuit, the superconductor must return to the nominal operating temperature, in other words it must cool down, to return to the superconducting state. This means that the superconducting cable cannot operate correctly immediately on the restoration of the short circuit, since it is necessary to wait for it to cool and return to the superconducting state.
Given the aforementioned problems in a network using conventional cables and coaxial superconducting cables, the Applicant has realized that the problems can be resolved, or in any case diminished, by making the electrical behavior of the coaxial superconducting lines in the presence of overcurrents substantially equivalent to the behavior of analogous lines using conventional cables.
In greater detail, the Applicant has realized that the electrical value of the superconducting line which is most important for the problems stated previously is the inductive reactance of the cables which constitute it, this value being significantly lower in coaxial superconducting cables than in conventional cables.
Even more particularly, the Applicant has found that a solution to the reported problems is obtained by raising the value of the inductive reactance of the coaxial superconducting cable to a value equal or close to that of a conventional cable in the same operating conditions.
An embodiment of the present invention comprises the connection in series with the coaxial superconducting cable of an inductive element having a value of reactance such that the total value of reactance (for the cable and the inductor) is made equal or close to that of a conventional cable for the same connection.
The Applicant has found that this solution makes it possible to obtain complete compatibility of the superconducting line with a conventional network.
According to another aspect of the present invention, the Applicant has also found that, in order to prevent the superconducting cable from overheating when, in, the presence of overcurrents, the superconductor passes from the superconducting state to the state of normal conduction, the cable has to be provided with an additional current path in parallel with the superconducting cable.
The Applicant has further noted that when the teachings of the invention are applied it becomes unnecessary to provide the network comprising superconducting lines with protective devices which are different (in other words capable of withstanding and stopping higher currents) from those used for the similar network comprising conventional lines.
In a first aspect, the present invention relates to an electrical power transmission network comprising: interconnecting nodes of the network and connecting lines between said nodes; a coaxial superconducting cable with which is associated a first reactance, connected between two nodes of said network; characterized in that it also comprises at least one inductive element, with which is associated a second reactance, connected in series with said coaxial superconducting cable.
Preferably, the sum of said first reactance and said second reactance is substantially equal to a third reactance whose value is substantially equal to the reactance of a conventional cable suitable for such a connection.
In particular, said at least one inductor comprises a superconducting cable, and may also comprise a core.
said at least one inductor is located at one end of said coaxial superconducting cable, or alternatively comprises two parts, of which one is located at one end of said superconducting cable and the other is located at the opposite end.
In one embodiment, said coaxial superconducting cable is of the multiple-phase type, and comprises at least one inductor connected in series with each phase of said coaxial superconducting cable.
In an advantageous embodiment, said coaxial superconducting cable comprises a support of conducting material and, in an alternative embodiment, a support of composite material.
In a second aspect, the present invention relates to a method for installing in an electrical power transmission system a connection using a coaxial superconducting cable, characterized in that it comprises the following steps:
determining the reactance of a conventional cable suitable for said connection;
installing said coaxial superconducting cable having a predetermined reactance;
increasing the reactance of said coaxial superconducting cable, in such a way that said reactance of said superconducting cable is substantially equal to the reactance of said conventional cable.
In particular, the step of increasing the reactance of said coaxial superconducting cable comprises the step of connecting in series with said coaxial superconducting cable an inductive element, preferably made from a superconducting material.
Advantageously, according to another aspect of the present invention, the method also comprises the step of associating with said coaxial superconducting cable a parallel conducting path of predetermined resistance, so that the short-circuit current is distributed between said superconducting cable and said conducting path in such a way that the maximum temperature reached by said coaxial superconducting cable is lower than the minimum temperature between the critical temperature of the superconducting material and the boiling point of the coolant fluid at the minimum working pressure of the fluid.
In a third aspect, the present invention relates to a method for replacing, in an electrical power transmission system, a conventional cable connection with a coaxial superconducting cable connection, comprising the following steps:
removing said conventional cable;
installing said coaxial superconducting cable; characterized in that it additionally comprises the step of increasing the reactance of said coaxial superconducting cable.
Preferably, the method additionally comprises the step of:
determining the reactance of said conventional cable;
increasing the reactance of said coaxial superconducting cable in such a way that the reactance of said coaxial superconducting cable is substantially equal to the reactance of said conventional cable.
In particular, the step of increasing the reactance of said coaxial superconducting cable comprises the step of connecting in series with said coaxial superconducting cable an inductance, preferably made from superconducting material.
Advantageously, the method also comprises the step of associating with said coaxial superconducting cable a parallel conducting path in such a way that the maximum temperature reached by said coaxial superconducting cable is lower than the minimum temperature between the critical temperature of the superconducting material and the boiling point of the coolant fluid at the minimum working pressure of the fluid.
In a fourth aspect, the present invention relates to a thermally insulated terminal for connection between a multiple-phase cable and an electrical installation at ambient temperature, said cable comprising, for each phase, at least one coaxial unit having a phase superconductor, a coaxial return superconductor and an interposed layer of electrical insulation, and also thermal control means for maintaining said superconductors of each of said coaxial units in the superconducting state, said terminal being characterized in that it comprises an inductor connected in series with each phase superconductor.
Preferably, the terminal comprises:
at least one casing,
cooling means,
a live current lead for each phase superconductor, having a corresponding phase connector for connection to said installation at ambient temperature, said current lead being provided with a resistive conductor between the phase superconductor and said connector of the current lead, the areas of connection between said resistive conductors and said phase superconductors being located inside the casing.
Preferably, the terminal comprises:
a single return current lead provided with a single resistive return conductor, with an upper end connected to a return connector for connection to the installation at ambient temperature;
connecting means made from a superconducting material between said return superconductors and said single resistive return conductor, the area of the junction between said connecting means made from a superconducting material and said single resistive return conductor, and at least said connecting means between the return superconductors and said single resistive conductor, being inside the casing and being at a temperature below the critical temperature corresponding to the superconducting state owing to the presence of said cooling means.